Seed-Mediated Growth and Advanced Characterization of Chiral Gold Nanorods

Abstract

The controlled growth of gold nanostructures with complex shapes and reduced symmetry, exemplified by chiral gold nanorods and nanoparticles, is one of the most dynamic fields of nanochemistry. A timely summary of underlying concepts, including growth mechanisms and redefined chirality measures, would further promote this research area. In this perspective, we aim to establish qualitative connections between the chiral shapes and growth conditions, specifically for the seed-mediated synthesis of chiral gold nanorods as a convenient case of chiral morphogenesis. The crystallographic and morphological features of achiral nanorods used as seeds, the experimental conditions during chiral growth, and the symmetry of the chiral inducers, can all be exploited to obtain nanorods with intricate chiral shapes. Chirality characterization (such as electron tomography techniques) and quantification (including chirality measures) emerge as critical aspects to comprehensively explore and understand such structures, enabling optimization of their geometric and optical features. We conclude by discussing relevant challenges to be addressed toward a better controlled synthesis of chiral plasmonic nanostructures.

Keywords: chiral gold nanorods; chirality measures; electron tomography; helicity; seed‐mediated growth.

Figure 1

a) Schematic view of the seed‐mediated synthesis of chiral Au NRs from anisotropic seeds. During symmetry breaking and anisotropic growth, isotropic seeds evolve into faceted NRs, which are subsequently used as seeds to grow chiral NRs. Adapted with permission from ref. [25] (Copyright 2023 The Authors). b) Possible scenarios for the seed‐mediated growth of single‐crystal (cube, octahedra, nanorods) and monotwinned (nanotriangle) seeds into chiral NPs with different types of anisotropy, protruding facets and wrinkles, and related Gibbs free energies. Au NRs can evolve into either twisted or wrinkled NRs, octahedra into 432 Helicoid III NPs, mono‐twinned nanotriangles into triangles with protrusions resembling propeller blades, whereas chiral growth of nanocubes leads to 432 helicoids I and IV. Twisted and wrinkled NRs, triangles with protrusions resembling propeller blades, 432 helicoids I and IV, and 432 helicoid III models are adapted with permission from refs. [6, 17, 26, 35], respectively (Copyright 2023 The authors, Copyright 2020 The Authors, Copyright 2022 The Authors, and Copyright 2018 Nature Publishing Group, in order).

Figure 2

a) Evolution of the optical activity of Au NRs during the growth of chiral features via successive HAuCl₄ additions in the presence of L‐cys. b) High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image showing the twisted morphology resulting from Au NR overgrowth after nine successive HAuCl₄ additions. Inset: single chiral Au NR electron tomography reconstruction. c) HAADF‐STEM image (top) and visualization of the 3D reconstruction (bottom) of an Au NR obtained after one chiral growth step. The white dashed lines indicate the location of {520} facets. Scale bar: 10 nm. Adapted with permission from ref. [25] (Copyright 2023 The Authors).

Figure 3

a) Schematics (top) showing the growth of a left‐handed penta‐twinned Au NR from elongated decahedra, using glutathione (GSH) as chiral directing agent and a representative SEM image of the synthesized chiral Au NRs (bottom). b,c) SEM images of penta‐twinned chiral Au NRs obtained in the presence of increasing concentrations of CTAB (b) and I⁻ (c). Reproduced with permission from ref. [31] (Copyright 2023 The Authors).

Figure 4

a) Molecular dynamics simulations of a helical micelle formed by BINOL and CTAC in water (top), and absorbed onto an Au cylinder (bottom). b–e) Spectral evolution of the anisotropy factor (b), electron tomography reconstructions (c, d, e‐left panels), and selected orthoslices (c, d, e‐right panels) of chiral Au NRs with increasing chiral shell thickness: 22 nm (red spectrum), 41.5 nm (blue spectrum), and 73 nm (purple spectrum). Scale bars: 50 nm. Reproduced with permission from ref. [18] (Copyright 2020 The American Association for the Advancement of Science).

Figure 5

a) Schematic description of how a complicated chiral structure can be abstracted into eight coordinates. The center of mass of the NP is assigned as the primary center of mass. Then, the NP's farthest surface point is designated as the z‐axis, and the x‐axis is assigned as the farthest in the xy plane. The coordination system segments the NP into eight pieces, generating their centers of mass as the secondary centers of mass. The coordinates of the secondary centers of mass can be used for chirality measure calculations. Reproduced with permission from ref. [45] (Copyright 2019 American Chemical Society). b) A 3D model of a rod with a right‐handed helical shell, as an ideal example to illustrate helicity around a rod, where the local helicity can be calculated according to the radius (ρ) and inclination angle (α); c) Visualizations of the 3D electron tomography reconstructions of two Au NR enantiomers (left), and the corresponding helicity function H(ρ,α) plot (right) resulting in a total helicity H_total. Scale bars are 50 nm. Reproduced with permission from ref. [78] (Copyright 2022 American Chemical Society). d) 3D‐color volume renderings of the helicity maps of wrinkled Au NRs synthesized in the presence of (R) or (S)‐BINAMINE. Blue colors indicate left‐handed helicity, and red colors indicate right‐handed helicity. Reproduced with permission from ref.[78] (Copyright 2022 American Chemical Society).

Figure 6

Characterization of chiral wrinkled Au NRs synthesized using penta‐twinned or single‐crystal Au NR seeds via the chiral micelle‐directed method. a) Helicity function plot for the chiral Au NRs obtained from penta‐twinned (PT) and single‐crystalline (SC) achiral Au NRs. b) Schematic illustration of the wrinkle direction angle (blue) and wrinkle orientation angle (orange). c–f) Central orthoslices (i) and isosurface visualization (i–iv) of the 3D reconstruction and enlarged graphs of chiral PT (c) and SC (e) NRs, respectively, showing the features of wrinkle direction angles and the corresponding polar plots (d, f) of the dominant wrinkle orientation a function of the rotation of each particle around its major axis (d: PT NRs; f: SC NR). Reproduced with permission from ref.[81] (Copyright 2024 The Authors).

Figure 7

a) Schematic comparison of the working principles between electron tomography and SEEBIC. b,c) SEEBIC images of S ‐ and R ‐BINAMINE induced chiral Au NRs with helical wrinkles, respectively. d) Helicity quantification of an S ‐BINAMINE induced chiral Au NR ensemble, based on 14 overview SEEBIC images; e) Helicity quantification of an R ‐BINAMINE induced chiral Au NR ensemble, based on 12 overview SEEBIC images. More than 100 particles in each case were analyzed, and the averaged helicity showed nearly mirrored statistics, which, on the one hand, confirmed that different enantiomers led to mirrored nanostructures and, on the other hand, supported the feasibility and advantages of the SEEBIC technique. Reproduced with permission from ref.[83] (Copyright 2024 American Chemical Society).